Why don't you prevent bad code?

Because that would be a cat and mouse game. Better to make
people aware of the problem than to give them a false illusion
of safety.

We are working on a function to flag bad scripts and rate good
ones. In future we may also validate a safe subset of functionality.

Can I run my .js or .pde scripts?

Not atm. but it shouldn't be too hard to get working.

This sucks, eclipse/vim/emacs is way better!

Sure, but you have to install those. A common theme in online
courses is to spend the first lesson setting up the students local
development environment. If students aren't there by compulsion,
but attend voluntarily, you have set up a barrier and lost 90% of
your potential audience.

With pcs, you just give them a link and they're on their way.
The primary goal for pcs is to make it dirt simple for people
to start editing code while keeping some of the comforts of the
better editors. These include syntax highlighting, a tight
edit-run loop, (just press alt+r), code completion (ctrl+space)
and documentation within the completion menu.